docs: explain supporting v1 and v2 plugins

packages/www/src/docs/content/build/plugins/index.mdx

@@ -1499,3 +1499,39 @@ Use versions compatible with the OpenCode release you target and test the
 installed package, not only a workspace-linked copy. Because the plugin API is
 beta, publish compatible plugin updates when V2 entrypoints or contracts
 change.
+
+## Support V1
+
+A plugin can support V1 and V2 from the same package entrypoint. Default export
+one object with a V1 `server()` function and a V2 `setup()` function:
+
+```ts title="src/index.ts"
+import { Plugin } from "@opencode-ai/plugin"
+
+export default {
+  ...Plugin.define({
+    id: "example",
+    async setup(ctx) {
+      await ctx.tool.hook("execute.before", () => {
+        console.log("A tool is about to run")
+      })
+    },
+  }),
+  async server() {
+    return {
+      "tool.execute.before": async () => {
+        console.log("A tool is about to run")
+      },
+    }
+  },
+}
+```
+
+- V1 calls `server()` and uses the returned hooks.
+- V2 reads the default export's `id` and `setup()` (or `effect()` for Effect plugins), ignoring `server()`.
+- Keep each implementation on its own API; sharing an export does not translate V1 hooks into V2 hooks.
+- Spread `Plugin.define(...)` into the exported object so it type-checks the V2 definition separately from `server()`.
+
+The V1 object form is supported in OpenCode `1.18.29`. Older V1 releases may
+expect function exports instead; test the installed package with the oldest V1
+release you intend to support and with V2.